1. Field of the Invention
The present invention relates generally to a drill and, more particularly, to a drill suitable for drilling a thin plate.
2. Description of the Prior Art
A drill suitable for drilling a thin plate is disclosed, for example, in Japanese Patent Laid-open (Kokai) No. 58-149115. This drill has a protrusive leading part protruding from the central part of the front end of the body. This drill will be described briefly with reference to FIGS. 6 and 7.
Referring to FIGS. 6 and 7, the drill has a pair of major cutting edges, and a chisel edge 3 at the junction of the major cutting edges. Each major cutting edge consists of an inner major cutting edge 1 forming the radially inner portion of the major cutting edge, and an outer major cutting edge 2 forming the radially outer portion of the major cutting edge. The pair of inner major cutting edges 1 project diagonally relative to the outer major cutting edges 2 toward the chisel edge 3 to form a protrusive leading part. A primary flank 5a and a secondary flank 5b, and a primary flank 6a and a secondary flank 6b are formed respectively behind each inner major cutting edge 1 and each outer major cutting edge 2. The secondary flanks 5b and 6b are inclined respectively at acute angles relative to the corresponding primary flanks 5a and 6a to secure chip spaces behind the primary flanks 5a and 6a. Indicated at 4 are web thinnings and at 7 are margins formed in the circumference of the drill body. The point angle b formed between the pair of inner major cutting edges 1 is approximately 105.degree., and the outer corner angle a, namely, the inclination of the outer major cutting edge 2 to a plane perpendicular to the axis of the drill, is zero. The pair of inner major cutting edges 1 forming the protrusive leading part cut well into a workpiece to position and lead the drill securely and precisely, which is particularly advantageous in drilling a hole with a hand drill.
However, since the flank of this known drill consists of the primary flanks 5a and 6a and the secondary flanks 5b and 6b, the thickness of the back metal near the major cutting edges, particularly, around the primary flanks 5a and 6a, is very small, and hence the cutting edges have a comparatively low strength and are subject to chipping.